Performance Benchmark E.12.B.1
Students know common characteristics of stars. I/S
As astronomers study stars, there are a number of characteristics that can be investigated:
temperature, composition, luminosity, mass, motion, and more. Some characteristics are
directly observable (such as temperature and some motions), while others (such as mass)
require inference from other data. Of these characteristics, the most important are color,
temperature, mass, and luminosity.
Although most appear white to our eyes, most stars have a predominant color that is
dependent upon their surface temperature. The hotter the star, the more blue light it emits;
conversely, cooler stars emit more red light. The predominant “color” can also be outside
of the visible range of wavelengths, for very hot (> 20,000K) or very cool (<4500K)
stars. If different colors are emitted with each about the same intensity, the star will
appear white; this can occur for stars whose surface temperature is moderate. Note that
these temperatures, though much hotter than what we encounter in our lives, are still
quite small compared to the temperature at the star’s core (which can be tens to hundreds
of millions of degrees!)
Figure 1. Blackbody curves of three stars showing the relationship between
temperature and color of maximum brightness. (from
http://www.astro.washington.edu/larson/Astro101/LecturesFraknoi/astro101s
04.html).
The mass of a star, though much more difficult to measure, determines how the star
changes, including its rate of change and ultimate demise. As a matter of convenience,
stellar masses are compared to that of the Sun, rather than using kilograms. The Sun is 1
MSun (“one solar mass”), which is roughly equal to about 2 × 1030 kilograms. The
smallest stars are a little less than 10% of the mass of the Sun, or 0.08 MSun, while the
most massive star yet measured is about 150 MSun. The mass of the star will influence
most of its other properties, including diameter, temperature, and lifetime.
To learn more about how astronomers determine the mass of a star, go to
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970609f.html.
A star’s mass determines the strength of its gravitational attraction, which in turn
influences the temperature and density at the core of the star. These factors will impact
the rate at which hydrogen is fused into helium (and later, other nuclear reaction rates as
well), thus determining the star’s lifetime. The most massive stars exist for the shortest
amount of time, while the low-mass stars can last hundreds or even thousands of times
longer. Our Sun is expected to have a main sequence lifetime (when it is fusing hydrogen
into helium) of 10 billion years. A star with a mass of 15 MSun has a main sequence
lifetime of only 15 million years, whereas a star with 0.5 MSun has a main sequence
lifetime of 200 billion years.
Figure 2. This graph shows the main sequence lifetime of a star is related to the star's
mass (dotted line). The graph also shows how the star's luminosity is related to its mass
(solid line). (From http://btc.montana.edu/ceres/html/Rare/rareearthactivity1.htm)
The star’s mass will also determine how it will “die,” whether as a planetary nebula and
white dwarf (in the case of low mass stars), or as a supernova which leaves behind a
neutron star or black hole (the most massive stars). Humans have not been able to
observe stellar evolution directly, of course: rather, computer and theoretical models are
supported by observations of thousands of individual stars.
To learn more about stars’ lifetimes and the main sequence, go to
http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html.
A star’s diameter is also determined by its mass – more massive stars, during their main
sequence period, have larger diameters. The diameter and temperature together determine
the star’s luminosity, or the total amount of energy emitted by the star every second. The
luminosity of stars ranges from about 0.0001 to more than 100,000 solar units (or LSun).
Unfortunately, we cannot measure luminosity directly – how bright the star appears to us
depends upon its distance as well as its luminosity.
To learn more about a star’s luminosity, go to
http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_luminosity.html.
A star’s luminosity and temperature are frequently plotted on a graph called the
Hertzsprung-Russell, or H-R, Diagram (named after two astronomers who independently
developed it). Temperature (or sometimes its spectral class, which is related) is plotted on
the horizontal axis, with the values decreasing to the right. Luminosity is plotted on the
vertical axis, using a logarithmic scale. When the corresponding values for a large
number of stars are then plotted on this graph, groupings of stars can be easily identified,
including the main sequence.
Figure 3. The H-R Diagram shows different groupings of stars
that have been interpreted as different stages of a star's lifetime.
(From
http://cse.ssl.berkeley.edu/segwayed/lessons/startemp/l6.htm)
To learn more about the H-R Diagram, go to
http://aspire.cosmic-ray.org/labs/star_life/hr_diagram.html.
Performance Benchmark E.12.B.1
Students know common characteristics of stars. I/S
Common misconceptions associated with this benchmark:
1. Students incorrectly think that all stars are white.
Stars emit all colors of light, but not necessarily the same amount of each. Depending
upon the star’s temperature, it may emit more of one color of light than of other colors. In
extreme cases, this color can even be viewed from Earth (for example, the star Betelgeuse
in the constellation Orion can appear reddish).
To learn more about the different colors of stars, go to
http://www.madsci.org/posts/archives/may97/859923240.As.r.html.
2. Students incorrectly think that blue corresponds to lower temperatures and red
corresponds to higher temperatures.
When we observe the human body and the technology around us, we see that cold is
represented by blue and hot is represented by red. Our skin takes on a bluish tint at
extremely cold temperatures and is reddened by sunburns and exertion, both of which are
seen more frequently at hotter temperatures. Likewise, the temperature controls in our
cars, homes, and the like use blue for cold and red for hot. However, the emission of light
works opposite of this: objects at lower temperatures emit more red light and objects at
higher temperatures emit more blue light. This phenomenon can also be seen in
chemistry, when using a Bunsen burner, where the hottest part of the flame is blue.
To learn more about how temperature relates to color, go to http://amazingspace.stsci.edu/resources/qa/ems.php.p=Astronomy+basics%40%2Ceds%2Castronomybasics.php&a=%2Ceds (in particular, see Questions 5 and 6).
3. Students incorrectly believe that more massive stars live longer than less massive
stars.
Since more massive stars contain more hydrogen, the possibility that it will take longer
for that hydrogen to be fused into helium seems perfectly reasonable. However, this is not
the case. More massive stars have a larger gravitational attraction pulling the gas particles
together, resulting in higher temperatures and pressures in the core. This leads to a higher
rate of fusion (the transformation of hydrogen into helium), so the hydrogen is depleted
faster than in a less massive star. The more massive stars therefore have shorter lifetimes
than less massive stars.
To learn more about how a star’s mass relates to its lifetime, go to
http://www.astronomynotes.com/evolutn/s2.htm.
Performance Benchmark E.12.B.1
Students know common characteristics of stars. I/S.
Sample Test Questions
1. Rank the order of a blue, a white, and a red star from coldest to hottest.
a. red  blue  white
b. red  white  blue
c. blue  red  white
d. white  blue  red
2. The color of star depends on
a. where the star is located in the galaxy.
b. the position of the star in its constellation.
c. the temperature of the star.
d. the composition of the star.
3. Star C will have a lifetime of 10 million years, while star D will have a lifetime of
300 million years. What can you say about the masses of these stars?
a. Star C has the greater mass.
b. Star D has the greater mass.
c. Stars C and D have about the same mass.
d. You cannot determine star mass from lifetime.
4. Which of the following determines most characteristics and future events of a
star’s existence?
a. size (diameter)
b. temperature
c. color
d. mass
5. Using the following diagram, what can you say about the relationship between a
star’s surface temperature and it total energy output (luminosity)?
Figure 4. From
http://www.fas.org/irp/imint/docs/rst/Sect20/A5.html
a. The lower a star’s surface temperature, the greater the star’s total energy
output.
b. The greater a star’s surface temperature, the greater the star’s total energy
output.
c. Total energy output decreases as the star’s surface temperature increases.
d. Total energy output is not directly related to the star’s surface temperature.
Performance Benchmark E.12.B.1
Students know common characteristics of stars. I/S.
Answers to Sample Test Questions
1.
2.
3.
4.
5.
(b)
(c)
(a)
(d)
(b)
Performance Benchmark E.12.B.1
Students know common characteristics of stars.
Intervention Strategies and Resources
The following list of intervention strategies and resources will facilitate student
understanding of this benchmark.
1. Amazing Space’s “Star Light, Star Bright”
This website explores the electromagnetic spectrum through several interactive
animations. In particular, the activities “Heating Up” and “Stellar Encounters” focus on
the relationship between a star’s temperature and color. The program can be found at:
http://amazing-space.stsci.edu/resources/explorations/light/
2. Sloan Digital Sky Server
The SDSS contains two detailed activities relating star color to temperature through the
use of real astronomical data. The basic activity can be found at
http://cas.sdss.org/dr5/en/proj/basic/color/ and an advanced one is at
http://cas.sdss.org/dr5/en/proj/advanced/color/.
3. Windows to the Universe “ExploraTour: A Peek into the Lives of Stars”
Although this web-based tour includes much more than just light and temperature, there
are several pages of interest. Start with the link “Colors of the night sky” at:
http://www.windows.ucar.edu/tour/link=/cool_stuff/tourstars_1a.html.